Allow const fn pointers to arbitrary memory address

Code

const FOO: unsafe extern "C" fn() = unsafe { core::mem::transmute(0x2123210 as *const ())};

fn main() {}

Current output

error[E0080]: it is undefined behavior to use this value
 --> src/main.rs:1:1
  |
1 | const FOO: unsafe extern "C" fn() = unsafe { core::mem::transmute(0x2123210 as *const ())};
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered 0x2123210[noalloc], but expected a function pointer
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
  = note: the raw bytes of the constant (size: 8, align: 8) {
              10 32 12 02 00 00 00 00                         │ .2......
          }

Desired output

successful build

Rationale and extra context

I'm developing a library for embedded environment. The microcontroller I'm working with features ROM filled with preflashed code.
I'd like to create a listing of the functions there, similarly to what the C library does, and put them as consts. This is arguably not UB at all, because it's absolutely certain that such functions exist at given addresses.

Other cases

No response

Anything else?

No response

Reproduce the example in src/main.rs and inspect the compiler's handling of const evaluation for function pointers and transmute. The work is complete when the shown constant compiles successfully for an arbitrary address without introducing an invalid-value diagnostic.